The invention relates to a process for producing a tangled fibre material from cut glass fibre bundles and a polymer-based binder as the starting product for the production of glass fibre-reinforced plastic mouldings and to an apparatus for performing this process.
The invention mainly relates to the processing of cut glass fibres, but can be used in the same way in connection with other inorganic fibres or synthetic organic fibres, such as carbon fibres, aramide fibres or polyester fibres. To the extent that reference is made to glass fibres, they can also be replaced by the aforementioned fibres acting in the same way.
Cut glass fibres are processed to a considerable extent for glass fibre-reinforced plastic mouldings. Problems occur when processing the glass fibres to a tangled fibrous material, because the glass fibres at least compared with organic fibres are comparatively flexurally rigid. In addition, the glass fibres are cut to length from multifilament fibre strands, so that glass fibre bundles are obtained, within which the individual glass fibres are parallel and closely juxtaposed. It is difficult to process the glass fibre bundles to tangled individual fibres. In practice, tangled fibre material formation takes place either by spreading out the fibres to form a fleece and impregnating the latter with a liquid synthetic resin, or the fibres are processed in liquid phase to a suspension, which is processed by a stirring or mixing movement to give a tangled fibre material. Here again, either liquid synthetic resins, or pulverulent binders are used in conjunction with an aqueous suspension. In the latter case, the suspension is dried after forming a tangled fibre fleece.
It has also been proposed (P 36 04 888.7) to add wetting agents in a proportion of max 20% by weight, so as to obtain a moist, but still free flowing material, which can be packed as an intermediate in plastic bags and so supplied to further processors or it can be processed in a heated belt press to preshaped plate or sheet material. The desired glass fibre-reinforced plastic mouldings can be produced from these intermediates.
The aim underlying the present invention results in providing a process and an apparatus for the production of tangled fibre materials from cut glass fibre bundles as a starting product for the production of glass fibre-reinforced mouldings, which operate on a dry basis.
According to the invention, the glass fibre bundles are vortexed and filamented in a turbulent air flow and the binder is added in powder form at least during vortexing.
Practical tests have revealed that the otherwise markedly direction-oriented and adhering glass fibre bundles can be opened up, i.e. filamented to a tangled fibrous material using the inventive compressed air process, in which the short single fibres are completely individualized and assume a completely irregular arrangement and distribution. It is possible in this way to produce from the cut glass fibre bundles, a voluminous wadding-like fibrous material, which is characterized by a maximum disorientation of the individual glass fibres.
In connection with the aforementioned production of the tangled fibre or fibrous material, the plastic binder is simultaneously added in powder form, because during the opening up of the glass fibres by whirling or vortexing in the compressed air flow and simultaneous introduction of the binder powder, an excellent, homogeneous mixture is obtained, in which the binder grains adhere to the glass fibres or are held in homogeneous distribution in the wadding or cottonwool-like tangled fibrous material. Advantageously the binder is fed into the air flow together with the glass fibres. During vortexing, due to the different structure of the glass fibres and the binder, (fibres or powder granules) they are accelerated differently and in this way the binder particles are virtually shot into the glass fibres. A completely homogeneous thorough mixing is not achieved if there is initially a vortexing of the fibres, followed by the addition of the binder and then an attempt is made to bring about mixing. Such a mixing would at the best merely be of a local nature.
As a result of the inventive procedure there is firstly a mechanical anchoring of the binder between the glass fibres, so that the binder granules or powder particles are surrounded by glass fibres and are secured between them. The mechanical anchoring can also be improved in that the pulverulent binder is supplied with an angular and rough surface configuration. According to a preferred embodiment, during vortexing, the components are electrostatically charged, which brings about an electrostatic adhesion of the components.
Advantageously the formulation of the binder powder falls within the following ranges: Thermoplastic powder 50 to 90% by weight, carbon black 0 to 15% by weight, antioxidants 0 to 5% by weight and miscellaneous, such as mineral fillers, particularly chalk, talc or the like 0 to 30% by weight.
The tangled fibrous material obtained in accordance with the invention which, as stated, has a cottonwool-like consistency, is preferably compressed to a felt-like structure. In this form, the tangled fibrous material can be handled for further processing purposes, e.g. it can be packed into film or sheet bags and transferred to the processor, or it can be immediately further processed after fleece production and using pressure and heat to a sheet or plate material to serve as an intermediate. Compression can in particular be brought about in that the packing of the tangled fibrous material takes place in bags under vacuum. To the extent that the binder has been vortexed with the fibres, this largely eliminates a phase separation of the fibres and binders during storage and transportation.
An apparatus for performing the process is initially characterized by a vortex chamber with venting and a compressed air line feeding into the same, as well as at least one feeding device for the glass fibre bundles and binder powder.
The glass fibres are fed by a feeding mechanism into the compressed air line and are entrained by the compressed air flow. The latter is given a strong, turbulent eddy flow in the following vortex chamber, the glass fibres being individualized and brought into a loose tangled form. Alternatively, the glass fibres can be fed directly into the vortex chamber.
Preferably, the apparatus is also characterized by at least one feeding mechanism for the binder powder, so that within the vortex chamber and in a single operation, not only are the glass fibres opened up to form a tangled fibrous material, but simultaneously mixing with the dry binder takes place.
According to a further advantageous development of the inventive apparatus, the compressed air line has a mouthpiece issuing into the vortex chamber and which is movable in varying direction with respect to the vortex chamber. The mouthpiece can, for example, be moved at right angles to the flow axis in an oscillatory movement or in a movement rotating about the flow axis, so that, within the vortex chamber, there is a constantly varying eddy flow. This permits a particularly effective and rapid production of the tangled fibre material.
According to a further development of the invention, the vortex chamber is constructed as a container with a discharge opening extending over the container cross-section and in that a discharge gate is provided which is introduced into the container from the side opposite to the discharge opening and which can be moved up to the latter. By the discharge gate, the tangled fibrous material collecting in the container can be discharged in simple manner.
According to a further feature of the inventive apparatus, a compression mechanism is connected to the discharge opening in order to produce the precompressed felt from the tangled fibrous material. This compression mechanism can advantageously be formed by the discharge gate and a counter pressure plate spaced from the discharge opening. Thus, the tangled fibrous material is preshaped to a felt immediately following discharge from the vortex chamber.
It is possible to connect to the compression mechanism either a heated belt press for further processing of the compressed glass fibre belt to glass fibre-reinforced plastic plates as an intermediate for the production of mouldings, or a device for packing the glass fibre felt in film packs. In the first case, handleable glass fibre-reinforced plastic plates are produced as an intermediate, which can be fed into a heated moulding press immediately following their production, or can be supplied to a further processor. In the second case, the felt can be packed in the form given to it. The packaging film is preferably made from the same material as the matrix (thermoplastic) of the felt, or from a material compatible therewith, so that it can be concomitantly processed during moulding production.
During the further processing of the inventively produced tangled fibre material to glass fibre-reinforced plastic mouldings, it has been found that compared with products produced in a conventional manner, there is an increase both in the modulus of elasticity and in the bending strength. The reason for this is doubtless the much better opening up of the glass fibre bundles and the more homogeneous mixing with the dry binder. In particular, it is possible to process higher-grade, so-called technical thermoplastics with a better thermal stability than the hitherto regularly used polypropylene. In particular, in the case of the process according to the invention, the individual components can be rapidly changed in small batches or charges, or the quantity thereof can be modified, so that the matrix (thermoplastic) and reinforcement type can at any time be adapted in optimum manner to the particular requirements for mouldings, e.g. on changing the moulds for producing the mouldings. The inventive process offers the possibility in connection with the production of glass fibre-reinforced products of reduced capital expenditure and energy costs, particularly as a single vortex chamber can be positioned downstream of several different moulds.
An important advantage of the felt or cottonwool-like intermediate produced by the inventive process is that through the tangled form of the individual fibres and not only the bundles, as in the prior art, there is a better flowability of the heated product during further processing.
The invention also provides a product for the production of fibre-reinforced thermoplastic parts, which is characterized by a cottonwool-like felt, in which the individual fibres are virtually completely individualized and assume an irregular arrangement and distribution. In the case of a plate or sheet-like semifinished product for the production of fibre reinforced thermoplastic parts, as well as a fibre-reinforced thermoplastic moulding, the individual fibres are virtually completely individualized and assume an irregular arrangement and distribution.
Following on to the inventive process stages, the semifinished product and moulding can fundamentally be further produced in known manner.
The invention is described in greater detail hereinafter relative to non-limitative embodiments and the attached drawings.